1. Field of the Invention
The present invention relates to a catalyst structure, such as those used in monopropellant reactors and those used to oxidize the combustion products from an automobile exhaust, and also to a method of making such catalyst structure.
2. Description of the Prior Art
Catalytic reactors are used in monopropellant rockets for attitude control of spacecraft, and in that application have to operate in pulse-mode, with possibly more than one million cycles per engine. Catalytic reactors also being considered for use as catalytic exhaust convertors for automobiles, where the catalytic reactor becomes exposed to pressure pulsations each time a cylinder exit valve of the automobile engine opens. Thus the number of pressure pulsations throughout the lifetime of the catalytic exhaust converter would be considerably more than one million.
Catalysts are quite often provided in the form of pellets or granules, which are packed into a container to make a catalyst bed through which the product to be reacted flows (e.g. a hydrazine monopropellant or automobile exhaust gases). One problem associated with such catalyst beds is that of low thermal conductivity. Since the granular or pelletized catalyst usually consists of an active metal deposited on an oxide ceramic carrier, there is low thermal conductivity caused by the insulating properties of the ceramic carrier and by the small contact area between the granules. This may result in hot spots in the bed, high thermal gradients through the bed, and in the case of a monopropellant reactor deep penetration of liquid, unvaporized propellant into the catalyst bed. Another problem associated with low thermal conductivity in the catalyst bed is that of ignition delays and pressure spikes associated with cold starts of a monopropellant reactor. To alleviate this, it is required to heat the catalyst bed, and of course lower thermal conductivity makes this more difficult.
Yet another problem associated with prior art catalyst beds is that the particles making up the catalyst bed tend to shrink because of high temperature and also because of attrition and spalling. When a small void is formed in the catalyst bed, the catalyst granules become free to move about and achieve significant speeds with each pressure pulse which passes through the bed. This results in accelerated attrition by abrasion and breaking of granules. In addition to catalyst granular motion caused by pressure pulsations, the catalyst bed is also subject to external vibrations of the catalyst reactor due, for example, to vibrations of a launch vehicle in a space application, or due to road vibrations in the case of an automobile. It has been attempted in the prior art to hold the granules in place by installing a spring loaded piston to compensate for the decrease in catalyst bulk volume. However, such springs often lose their resilience after prolonged exposure to high temperature at which catalytic reactors usually operate.
The prior art contains a variety of suggestions as to the various substrates that may be used for the catalyst material, such as bars, balls, chain, mesh, plates, saddles, sheet, tubes, wire, steel wool, etc. There have also been suggestions in the prior art that a foam metal may be suitable as a catalyst material. However, to the best knowledge of the applicant, most of the prior art disclosures relating to catalysts deal with the effectiveness of the catalyst material itself and make little distinction as to the advantages or effectiveness of the physical arrangement or construction of the catalyst or the structure associated therewith. Typical of such prior art disclosures are those contained in the following U.S. patents: Houdry, U.S. Pat. No. 2,742,437; Sill, U.S. Pat. No. 3,135,703; Rose, U.S. Pat. No. 3,147,592; Cohn et al, U.S. Pat. No. 3,197,955; Leak et al, U.S. Pat. No. 3,231,520; Webb, U.S. Pat. No. 3,298,182; Grant, Jr., et al, U.S. Pat. No. 3,303,651; Hall, U.S. Pat. No. 3,377,140; Talsma, U.S. Pat. No. 3,397,154; Tope et al, U.S. Pat. No. 3,421,826; Kearby, U.S. Pat. No. 3,423,194; Innes, U.S. Pat. No. 3,438,721; Keith et al, U.S. Pat. No. 3,441,381; Barber et al, U.S. Pat. No. 3,495,950; Sowards, U.S. Pat. No. 3,502,596; Friant et al, U.S. Pat. No. 3,507,627; Berger, U.S. Pat. No. 3,533,753; Keith et al, U.S. Pat. No. 3,565,830; and Keith et al, U.S. Pat. No. 3,597,165.